Actin polymerization on inside out erythrocyte membrane vesicles depends upon the presence of a membrane protein. We are trying to identify that protein of proteins. The purity of the actin is crucial in these experiments and we are investigating what impurities modify the actin-membrane interaction. The effects of several lectins on the MgATP dependent shape change of erythrocyte ghosts have been studied. Concanavalin A inhibits the shape change process whereas the Anti-H lectin (Ulex europeus) accelerates it. The biochemical and biophysical basis of these effects are under study. BIBLIOGRAPHIC REFERENCES: Sheetz, M.P. (1977) Cation effects on Cell Shape, proceedings ICN-UCLA Symposium on Cell Shape and Surface Architecture. J. Supramolec. Struct. Vol. 5, in press. Sheetz, M.P. and Singer, S.J. (1977) On the Mechanism of ATP-Induced Shape Changes in Human Erythrocyte Membranes. J. Cell Biol. (June issue), in press.